$A=\left[\begin{array}{rr}0 & 4 & 6 \\ 5 & 2 & 7 \\9 &1 &3\end{array}\right]$ $A_{2,1}=$
Answer: Background An $m\times n$ matrix has $m$ rows and $n$ columns. $A=\left[\begin{array}{rr}A_{1,1} & \cdots & A_{1,n} \\\\\vdots \ & \ddots & \vdots \\\\A_{m,1} &\cdots &A_{m,n}\end{array}\right]$ Therefore, the entry $A_{{c},{d}}$ is located on row ${c}$ and column ${d}$. Finding $A_{2,1}$ $A_{{2},{1}}$ is located on row ${2}$ of $A$ : $\left[\begin{array}{rr}0 & 4 & 6 \\ {5} & {2} & {7} \\9 & 1 &3\end{array}\right]$ $A_{{2},{1}}$ is also located on column ${1}$ of $A$. $\left[\begin{array}{rr}{0} & 4 & 6 \\ {\text{5}} & {2} & {7} \\{9} &1 &3\end{array}\right]$ Therefore, $A_{{2},{1}}={5}$. Summary $A_{2,1}=5$